Aminopeptidase N (APN) is an exopeptidase that hydrolyses neutral amino acids from the amino (N)-termini of different proteins. In different cell types, APN is expressed as a soluble cytoplasmic enzyme and a membrane-bound ectoenzyme. This enzyme is found on the surface of diverse cell types including lung, kidney, intestine and brain cells of many animals (Kenny et al., 1987). The ectoenzyme form is attached to epithelial cells of intestinal brush borders and respiratory tracts of vertebrates by a hydrophobic N-terminal stalk (Kenny et al., 1987 and Takasaki et al., 1991). In insects, however, ectoenzyme attachment is via a glycosyl-phosphatidylinositol (GPI) anchor (Tomita et al., 1994; Garczynski and Adang, 1995; Luo et al., 1996a; Luo et al., 1996b; Luo et al., 1997a; and Luo et al., 1997b). GPI-anchored proteins are relatively mobile on the membrane surface and can be clustered in microdomains with other proteins and specific lipids. The base of the GPI-anchor interacts with the intracellular environment and has been implicated in physiological functions, intracellular sorting and transmembrane signaling (McConville and Ferguson, 1993).
In intestinal epithelial cells, APN is important for the final hydrolysis step of ingested proteins. APN also has several important physiological roles in other tissues. For example, APN is implicated in tumor cell invasion and inhibition of aminopeptidase activity can suppress tumor cell spread (Fujii et al., 1995). In brain cells, APN serves a role in the breakdown and inactivation of peptide neurotransmitters (Kenny et al., 1987). In bovine renal brush border membrane vesicles (BBMV), partially purified APN was found to be associated with a Na+-dependent amino acid co-transporter (Plakidou-dymock et al., 1993).
APN molecules function as adventitious receptors for viruses. Human, feline, canine, and porcine coronaviruses utilize APN as their cellular receptors (Delmas et al., 1992; Yeager et al., 1992; and Tresnan et al., 1996). Cells refractory to coronaviruses from a particular animal species can be made susceptible by expression of an APN cDNA from that species (Benbacer et al., 1997). Human APN was shown to mediate human cytomegalovirus infection by increasing virus binding (McLaughlin and Aderem, 1995). Human, porcine and feline APNs have been cloned and expressed in different cell lines (Delmas et al., 1992; Yeager et al., 1992; Kolb et al., 1996; and Tresnan et al., 1996). Each of these vertebrate APNs were expressed on the cell surface as the N-terminal stalked form and bound a coronavirus.
Isoforms of APN located in the epithelial cells of insect midguts bind specifically to Bacillus thuringiensis Cry1 δ-endotoxins. Toxin-binding APNs are reported for several lepidopteran species (see, e.g., Knight et al., 1994; Sangadala et al, 1994; Gill et al., 1995; Valaitis et al., 1995; Luo et al., 1996; and Yaoi et al., 1997). For example, Cry1Aa, Cry1Ab and Cry1Ac, but not Cry1C or Cry1E toxins bind to a purified 115 kDa APN from Manduca sexta (Masson et al., 1995). Also partially purified preparations of APN catalyze toxin-induced pore formation in membrane vesicles (Sangadala et al., 1994) and planar lipid bilayers (Schwartz et al., 1997).
Several APN isoforms have been purified and cloned from different insect species (see, e.g., Knight et al., 1995; Gill et al., 1995; Valaitis et al., 1995; Luo et al., 1996; Yaoi et al., 1997; Denolf et al., 1997; and Hua et al., 1998). However, there has been limited success in expressing insect APN cDNA in insect cells. The only example to date involved the expression of Plutella xylostella 105 kDa APN in Sf9 cells using a baculovirus vector (Denolf et al., 1997). While the transformed cells of this study produced APN localized to the cell membrane, the APN was unable to bind to B. thuringiensis Cry1A toxins. Further, Denolf et al. were unsuccessful in expressing two 120 kDa APNs from Manduca sexta using the same vector.
The complete structural and functional characterization of insect APN will require the successful expression of insect APN in insect cells. Successful expression of insect APN in insect cells as described in Luo et al. (1999) would also facilitate study of APN-toxin interactions, as well as provide a screening system for obtaining novel pesticide agents.